Anticipated Impacts on veteran's Healthcare: Gulf War Illness (GWI) is a chronic, multi-symptom illness characterized by fatigue, pain, gastrointestinal symptoms (particularly irritable bowel syndrome), and problems with cognitive function. Approximately 25-35% of the 1991 Gulf War Veteran population report symptoms consistent with GWI. There have been fairly modest advances in treatment of GWI, but the pathophysiology is poorly understood and there remains a critical need for identification of innovative, novel therapies. Data from conditions where the symptomatology is somewhat similar to GWI, such as chronic fatigue syndrome (CFS), suggests that the brain-gut axis is important in mediating many of the symptoms of chronic multisystem illness. In the case of CFS, there is evidence for a synergism between aberrant gut microbiota, mucosal barrier dysfunction, and altered mucosal immunity contributing to the multisystem pathology, including central nervous system dysfunction. Research shows that patients with CFS have undesirable alterations in gut microbiota and that pathogenic and non-pathogenic gut microbiota may influence symptoms remote from the gastrointestinal tract and even behavior in animals and humans. Project Background: Bacteria found in the human gastrointestinal (GI) tract play a significant role in maintaining health by providing energy, nutrients, and immunological protection. Intestinal dysfunction is a frequent complaint in GWI and CFS patients, and given the similarities between CFS and GWI, it is possible that the same gut abnormalities described in CFS may be responsible for the symptomatology in GWI. However, the gut microbiota structure in GWI patients has not yet been well characterized. Since interventions such as probiotics could favorably alter the gut microbiota, improve mucosal barrier function, decrease pro- inflammatory cytokines, and have the potential to positively influence not only gastrointestinal symptoms but also non-gastrointestinal symptoms in GWI veterans, an understanding of the gut microbiota structure and function in veterans with GWI is essential. Project Objectives: The objective of this pilot application is to characterize the gut microbiota of GWI veterans compared with healthy GW veterans. Project Methods: We propose a longitudinal, observational study using serial stool and blood samples in 50 subjects, 25 with GWI and 25 who are healthy controls over 8 weeks. Participants seeking care at the Madison VA as well as other veterans from Madison and the surrounding areas will be recruited. Inclusion criteria include veterans ages 35-75 who were deployed to the Persian Gulf for Operation Desert Storm or Operation Desert Shield. We will collect serial stool and blood samples to examine gastrointestinal microbiota using high throughput sequencing and biomarkers of inflammation and immunity. This work will be the first to comprehensively examine gut microbiota in Gulf War veterans. The results of this work will advance our understanding of the pathophysiology of GWI especially the role of gut dysbiosis in mediating GWI symptoms.